As is well known, the barrel of an artillery gun, such as a tank and the like, must be angularly displaced from the gunner's line of sight by an appropriate angle in order that the projectile strike the target at which the muzzle was aimed. The angle is a function of a number of factors including the distance to and motion of the target, the ejection speed of the projectile, cross winds and others. The vertical component of the angle is known as the "superelevation angle" while the horizontal component is known as the "lead angle".
Modern fire-control systems utilize a computer to calculate the superelevation angle and the lead angle on the basis of signals provided by a number of sensors and then automatically align a gunner's sight with respect to the axis of the gun barrel. Notwithstanding the sophistication of fire-control systems, the accuracy of the gun is reduced because of various and unavoidable errors which result from gun droop and thermal distortion of the gun barrel and other components. Systems which detect, measure and compensate for these errors are known generally as "muzzle reference systems".
One known muzzle reference system provides a diffuse light source mounted on the turret roof of a tank, a mirror mounted at the muzzle end of the barrel, a deflection prism and focusing lens arrangement incorporated in the gunner's sight and a reference circle superimposed on the reticle pattern of the gunner's sight lens. To compensate for error, the gunner sets the gun barrel to a predetermined position or angle and activates the light source. The mirror reflects a portion of the light beam generated by the light source through the prism and lens arranged on the gunner's lens. The magnitude and direction of the displacement of the beam with respect to the reference circle on the gunner's lens is a measure of the error. The gunner manually adjusts bore citing knobs to move the beam into the reference circle. While this system is adequate for some tank configurations, it requires a line of sight between both the light source and the mirror and the mirror and the gunner's lens--an option which is not available on all tank configurations. In addition, the diffuse light source may reveal the location of the tank during night use. A significant drawback of this system is that the error compensation process is time consuming because the barrel must be stationary during calibration and adjustments must be effected manually.
A modification of the above described prior art system involves replacing the light source and mirror with a collimator positioned at the end of the muzzle and aimed at the deflection prism in a reference position of the barrel. The operation of this system is essentially the same as the parent system and, accordingly, the modified system suffers substantially the same disadvantages. In addition, however, the collimator is a complex and fragile device which requires sophisticated mechanical design to withstand the high acceleration generated by a gun blast.
Another muzzle reference system provides a laser source mounted on the tank, a diverging lens mounted along the optical axis of the beam produced by the laser source, a beam splitter positioned along the optical axis, a retroreflector mounted at the end of the gun muzzle and an optical detector positioned adjacent the beam splitter. The diverging lens diverges the laser beam over an area at the gun muzzle compatible with expected errors. To compensate for error, the gunner sets the barrel at a predetermined position and activates the light source. The retroreflector reflects a portion of the incident beam back towards the beam splitter which, in turn, reflects the beam onto the detector. The detector produces an electrical output signal representative of the displacement of the gun muzzle from the reference position. A deformation equation must be used to transform the positional error to an angular error. This system is considered superior to the aforementioned systems in that it does not include large optical components on the muzzle, does not require a line of sight from the gunner's sight to the muzzle and the mode of operation is susceptible to a fully automatic configuration. However, the technique is severely handicapped by the requirement of complex equations which link the desired angular error to the measured positional error and which must include compensation for sun heating with and without cooling and so forth.
A still further system which may fall within this general category has been specifically developed for measuring gun droop and thus is not a complete muzzle reference system. In general, this arrangement provides a laser transmitter mounted on the turret, a mirror mounted at the muzzle end of the gun barrel and a receiver having a position sensing device also mounted on the turret. The transmitter directs a beam at the mirror which, at a predetermined position of the muzzle, reflects the beam towards an aperture in the receiver in which the position sensing detector is disposed. The receiver is of the form of a camera which focuses the beam onto the position sensing detector in the focal plane of the camera lens. As the gun muzzle bends,the angular motion of the beam causes the focused light on the detector to translate linearly. This system is superior to the aforementioned systems because it measures required angular displacement of the muzzle as opposed to positional displacement. However, this system provides only muzzle-to-mirror compensation and additional means are required to provide the mirror-to-line of sight error compensation necessary for a complete muzzle reference system.